A monitor unit has been utilized in combination with a programmable logic controller (PLC) in order to monitor a task performed by I/O devices connected to and controlled by the PLC. The monitor unit includes a display for displaying visual information prepared for informing the task being performed, a controller section for controlling to display the visual information, and a memory for storing the visual information to be selectively reproduced on the display. In order to design the visual information, an image design tool is provided for creating a source display code of the visual information. After completing the design of the visual information on the tool, the source display code is transferred to the monitor unit where the source display code is translated into a corresponding bitmapped image for reproduction of the visual information on the display. Normally, the visual information created on the tool includes a combination of characters and graphic images. The characters are realized as vector fonts by the image design tool or draw program with the use of a font file provided on a computer running the draw program, while the graphic images are realized in a vector-oriented format defined by the program. Therefore, the monitor unit is required to have additional character ROM for translating the characters into the corresponding bitmapped image as well as to have a capability of translating the graphic images in the vector-oriented format into the corresponding bitmapped image. The more the intended visual information becomes sophisticated with the inclusion of elaborate graphic images and the characters of different font sizes, the more the monitor unit is required to have the translation capability for rapid reproduction of the visual information on the display. To meet this requirement, the prior monitor unit has to incorporate a high speed CPU and a graphic processor, and therefore is realized at a considerably high cost.